1. Field of the Invention
The present invention relates to electrical heating of subsea pipelines. More particularly the invention relates to a power supply system for supplying electrical power to the pipeline.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressuring the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create more operational problems and be costly in terms of lost production.
The problem of lower temperatures in subsea pipelines has been addressed by a variety of heating methods, including electrical heating. Heating by a variety of electrical methods is well known in industry. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999). One electrical heating method is the pipe-in-pipe method. In one configuration of this method, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at bulkhead at the distal or remote end of a heated segment. Voltage is applied between the inner and outer pipes at the proximate or electrical input end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707, which is commonly assigned and hereby incorporated by reference herein. Anther configuration for electrical heating of subsea pipelines is the xe2x80x9csingle heated insulated pipexe2x80x9d method, in which only one pipe is insulated from seawater by an electrically insulating coating. This configuration is disclosed in commonly owned patent application Ser. No. 09/629,963, entitled xe2x80x9cApparatus and Method for Heating Single Insulated Flowlines.xe2x80x9d Other heating configurations include the earthed-current system developed in Norway and described in xe2x80x9cIntroduction to Direct Heating of Subsea Pipelines,xe2x80x9d overview by Statoil, Saga et al, February 1998.
In any form of electrical heating of pipelines a source of electrical power supplying at least a few hundred kilowatts is normally needed. The power often needs to be applied in a programmed sequence to achieve selected operating conditions. In some methods of electrical heating, such as the pipe-in-pipe method, it is desirable that the output of the power supply have very low harmonic content and be delivered with little or no direct current. Selection of a particular frequency may be an advantage. Conversion of power from three-phase to single-phase will often be necessary, especially in subsea pipeline applications, where power may be taken from an existing three-phase power grid on an offshore platform. What is needed is an efficient, versatile power supply system that can supply these needs.
A system including a variable frequency drive that may be electrically connected to a power bus and a Distributed Control System (DCS) is provided. A medium voltage drive will normally be employed. The output of the variable frequency drive is preferably coupled to a pipeline through an isolation transformed and a contactor or isolating switch. Control of the variable frequency drive may be provided by a DCS that is connected with the variable frequency drive through an Ethernet connection or is directly connected.